Help understanding sensor placement on multiplexing schematic

Other Hardware LEDs and Multiplexing
4

if i want to use capacitive sensor here

I would forget it. The capacitive sensor idea is not a very reliable thing to do, it is even less reliable when using a multiplexer on the input.
This is because :-

  1. It relies on a very high impedance input.
  2. Multiplexers do not work well if at all with high impedance inputs, there is too much cross talk between the channels.